The present invention relates to apparatus for digitizing the contour of a three-dimensional surface. The invention is particularly applicable for digitizing a dental surface, e.g., a ground tooth for preparing a dental crown, and is therefore described below with respect to this application.
When a tooth is in an advanced state of decay, it may become necessary to apply a crown. This involves grinding the tooth and then fixing the crown to it. If one or more teeth are missing or are to be removed, it may be necessary to insert a bridge, which includes a crown applied to each of the two teeth to be bridged and integrally formed with one or more artificial teeth to fill the space of the missing tooth or teeth.
The metal infrastructure for preparing a crown or bridge is conventionally produced by making a cast model of the respective part of the oral cavity, and then using the cast model for building the crown or bridge. Such a procedure is very time-consuming and requires a large number of visits by the patient to the dentist. Moreover, this procedure is greatly dependent on the skill and experience of both the dentist and the laboratory technician in making the cast model and the crown or bridge based on it, as well as on the precision of the equipment used.
Many systems have been proposed for automatically producing crowns and bridges by digitizing the contour of the tooth to be crowned. Some of the proposed systems are based on optical measurements, as illustrated for example in U.S. Pat. Nos. 4,742,464, 4,663,720, 4,611,288 and 4,575,805. Other proposed systems are based on the use of contact-type probes which are passed over the surface of the tooth to be crowned. Examples of the latter type system are illustrated in U.S. Pat. Nos. 4,182,312, 4,478,580, 4,411,626 and 3,324,546.
U.S. Pat. No. 4,182,312 discloses a contact-type dental probe in which an aluminum tray is fixed in the patient's mouth such that when the dentist applies the probe to the patient's tooth or gum tissues, an electrical circuit is completed, via the saliva in the patient's mouth, to the tray to enable the transducer to output the positional signals of the probe. However, since the contact of the probe with the surface being digitized is based on completing an electrical circuit to the tray via the subject's saliva, the results would be distorted by the thickness of the saliva film, which is significant when compared to the precision required for dental probes of this type. U.S. Pat. No. 4,478,580 discloses apparatus for treating teeth in which a rotary cutter head is provided with stress sensors so that both the shape of the tooth being ground, as well as the resistance to grinding, are sensed during the grinding operation.
Insofar as we are aware, apparatus for digitizing the contour of a tooth in order to automatically prepare a crown or a bridge has not yet gained widespread use despite the long felt need and search for such an apparatus.